Xenobiotic-metabolizing enzymes are responsible for metabolism and inactivation of all clinically used drugs. They are also involved in the metabolic activation or inactivation of toxins, mutagens and chemical carcinogens. Marked differences in levels of expression of these enzymes have been found in humans and these differences could contribute to interindividual differences in sensitivities to drugs and carcinogens. Variable gene expression could account for some differences in levels of expression of xenobiotic-metabolizing enzymes. Most of these enzymes are expressed in the liver and their genes are under control of different hepatocyte transcription factors. Several families of transcription factors are preferentially expressed in the liver and control liver-specific gene expression. Typically, in vitro techniques, including transfections of reporter gene constructs and DNA binding assays, are used to study gene regulation. However, it is difficult to directly demonstrate that results obtained using in vitro studies actually reflect gene expression in the intact animal. Studies to determine whether hepatocyte-enriched factors are involved in regulating gene expression in vivo can be done by using gene knockouts to disrupt expression of transcription factors and then determine the effects transcription factor loss on target gene expression. However, standard gene knockouts of transcription factors frequently results in either pre- or post-embryonic lethality. To circumvent this problem, mice lacking transcription factor expression are being produced using the Cre/LoxP conditional knockout system. Mice containing the recombination signal LoxP flanking the C/EBPalpha gene have been produced and exhibit normal phenotypes indicating that the LoxP sites do not interfere with gene expression. The Cre recombinase was introduced to adult mice containing the LoxP-modified C/EBPalpha allele using a recombinant adenovirus yielding 90% recombination and deletion of the C/EBPalpha gene in liver. This resulted in loss of expression of several target genes including the UDP-glucuronosyltransferase, UGT1, responsible for conjugation of bilirubin. Expression of other genes encoding xenobiotic- metabolizing enzymes are being analyzed.